Lipids

• Lipids are biological molecules made of carbon, hydrogen and oxygen which are only soluble in organic solvents such as alcohols.
• The main lipid types are triglycerides and phospholipids.
• Triglycerides are lipids made of one molecule of glycerol and three fatty acids joined by ester bonds formed in condensation reactions.
• There are many different types of fatty acids (over 70 different types) which vary in chain length, presence and number of double bonds.
• The presence of double bonds changes the name of the lipid, they can either be saturated or unsaturated.
• Saturated lipids such as those found in animal fats don’t contain any carbon-carbon double bonds.
• Unsaturated lipids which can be found in plants contain carbon-carbon double bonds.
• The presence of a double bond means that the molecule is able to bend, as a result unsaturated fats cannot pack together as tightly and are therefore liquid at room temperature.
• Triglycerides have a high ratio of energy storing carbon-hydrogen bonds to carbon atoms and therefore they are an excellent energy store.
• Triglycerides have a low mass to energy ratio meaning that they are a good storage molecule, with a lot of energy being stored in a small volume.
• Triglycerides are large and non-polar lipids and are insoluble in water, therefore their storage does not affect the water potential of cells.
• Phospholipids have one of the fatty acids of a triglyceride substituted by a phosphate-containing group.
• Phospholipids form micelles when they are in contact with water, making them polar.
• In an aqueous environment, being polar means a bilayer can be formed.
• The hydrophilic heads of the phospholipids can be used to hold at the surface of the cell surface membrane.
• Phospholipids can form glycolipids with carbohydrates which are important on the cell surface membrane for cell recognition.
• An emulsion test can be used to test for the presence of lipids.
• This is carried out by taking a completely grease free test tube and adding 2cm3 of the sample to be tested and 5cm3 of ethanol, shaking the test tube thoroughly to dissolve all the lipid in the solution, adding 5cm3 of water and shaking gently, a cloudy-white colour indicates the presence of a lipid, as a control repeat the experiment using water as the sample, the final solution should remain clear.